The technical field of this invention is that of nondestructive materials characterization. This includes inspection of materials for hidden objects and characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy-current sensors or electric field based or capacitive sensors. These nondestructive evaluation (NDE) techniques are applied across a wide variety of applications, ranging from manufacturing quality control to the detection of buried objects. Interrogation using electromagnetic fields is one technique that often proves to be of great use since it is sensitive to both geometric and electrical properties of materials. Additionally, many other non-electrical properties of interest can affect the measured electrical properties, such as cure state, fatigue, cracking, and temperature, which further the use of electromagnetic methods.
Although the full electromagnetic spectrum can be considered useful to some degree for NDE, here the focus is on fields that are sufficiently low in frequency to be considered quasistatic. A common magnetoquasistatic (MQS) measurement device used in NDE is the eddy-current sensor. Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks. Similarly, a common electroquasistatic (EQS) measurement technique uses capacitive sensors. This involves placing parallel plates on either side of a relatively insulating material, forming a capacitor, for which the admittance is dependent on the properties of the material.
Advances in these techniques have resulted in sensors which utilize spatially-periodic planar windings (MQS) or electrodes (EQS) which can be fabricated accurately using micro-fabrication, flex-circuit, or wire winding techniques depending on the sensor's size. Representative sensor geometries are described in U.S. Pat. Nos. 4,814,690 and 5,015,951. The periodic and planar nature of these sensors has allowed their terminal response to be modeled for layered materials, using semi-analytic methods. Their periodic nature also allows a means for affecting the cumulative depth to which a material is interrogated.